When users are signed-in to any of the commercial/consumer web workloads (e.g. Outlook®), and they navigate to a new browser tab to go to a different workload (e.g. Office365®), the workloads do not have a way to pick the right user identity. A typical result lands the user into an unauthenticated experience. Typically, in order to sign-in the user from there would require several redirects which is inefficient from a processing standpoint and also creates a poor user experience.
When dealing with just one IDP, things become fairly simple. A service may redirect to the login link of that one IDP and the logic will kick in to sign the user in. When more than one IDP is involved, a service cannot send the user to just one IDP because the service will not know which user must be signed in. Some current services provide single sign-on (SSO) functionality for user accounts, where a web browser may be configured to pass sign-in data (e.g. a previously created cookie) to the service when a browsing experience changes. However, such services are limited in that they are only configured to work with a single identity provider (IDP).